The invention relates to the field of system control technology, in particular to the substation control technology for high, medium or low voltage switchgears. It is based on a method, a computer program product, a computer program and a device for checking the plausibility of voltage transformers in an electrical switchgear as well as on a switchgear comprising such a device according to the pre-characterizing clause of the independent patent claims.
A power supply network comprises substations or electrical switchgears, in particular high or medium voltage switchgears, which are controlled by a distributed substation control system. The substations include primary or field devices, e.g. switches, drives, generators, transformers, voltage transformers and current transformers. The control system has, for example, field control devices and an operator station, which are connected to one another by means of various communication busses and bus couplers. The field control devices control, adjust, monitor and protect the field devices in the system.
Voltage transformers in substations measure the system voltages at prescribed measuring points in the switchgear system with a certain measuring inaccuracy. The measured voltage values are filtered, are scaled to primary voltage values of the system, are digitized if necessary, are recorded by the control system as voltage measuring signals and are presented on a display and/or are used for calculation purposes. Additional rate-of-change filtering for communication purposes can, for example, include time averaging by means of integration or an algorithm for deciding whether to update or to maintain and to transmit or not to transmit the voltage value. Both the measured values recorded by the voltage transformer and the parameters for filtering and scaling, i.e. the so-called parametrisation of the measured values, can be subject to errors. Primary errors in the voltage transformer itself occur, for example, due to defective parts or material fatigue. Errors in the parametrisation can be caused in the electronics by external influences, aging, drift, operator error or similar.
For protection purposes and for the synchro check, it is known to monitor the voltage transformer fuse and also to carry out local plausibility tests at an isolated point of the switchgear. The known plausibility tests, which are referred to a single measuring point, are a comparative test of current and voltage values, a check on the phase sequence of the phase voltages, a phase symmetry test or a voltage symmetry test. In the comparative test, serious inconsistencies between voltage and current values at the measuring point can be detected. When monitoring the phase sequence, inconsistencies between the time evolution of voltages of different phases at the measuring point can be detected. In the phase symmetry test, voltage maxima of all three phases at the measuring point are checked with respect to their phase position relative to one another and deviations from a maximum phase shift are detected. This test works only if not too large maximum values for the permissible asymmetry can be specified. Furthermore, this test is normally impossible when transducers are used for digitizing the voltages, as these provide, instead of the required instantaneous values, time-averaged RMS values of the voltage, power, frequency or phase angle. In the voltage symmetry test, all phase-to-phase voltages and the zero voltage are summed and deviations from the ideal value of zero are monitored. These local plausibility tests are relatively coarse and allow only a yes/no decision about the operativeness of the voltage transformer or the voltage transformers at the measuring point. On the other hand, a loss of measuring accuracy or an incorrect scaling cannot be detected.
At network control level, in a so-called xe2x80x9cstate estimationxe2x80x9d, plausibility tests on power and voltage values are also known in order, for example, to compensate for inconsistencies due to measurements at different points in time or due to measuring errors, However, such plausibility tests are not suitable for detecting accuracy or scaling problems with voltage transformers, because it is practically impossible to identify the true reason for the deviation. In addition, a state estimation is too elaborate, from the point of view of both the required computational power and the configuration, to be used just for the detection of voltage transformer problems in a substation.
The object of the present invention is to specify a method, a computer program product, a computer program a device and an electrical switchgear with such a device for the improved and simplified monitoring of voltage transformers in electrical switchgears. According to invention, this object is achieved by the features of the independent claims.
In a first aspect, the invention consists in a method for the plausibility checking of voltage transformers in an electrical switchgear, in particular a high or medium voltage switchgear, the switchgear being controlled by a substation or switchgear control system and measured signals from voltage transformers arranged at least two different measuring points being processed by the control system, the following steps for testing the plausibility of the voltage transformers being carried out: (i) recording by means of the control system of an instantaneous topology of the switchgear based on the existing electrical connections of primary devices and based on the instantaneous positions of switches; (ii) identifying, based on the instantaneous topology, at least one sub-area of the switchgear, which has at least two galvanically connected voltage transducers; (iii) within the sub-area, dividing the voltage transformers, based on their measured signals, into groups, all of the measured signals in each group coinciding with one another within the tolerated measuring inaccuracies; and (iv) generation of an indication or alarm signal by the control system if more than one group is present. By means of the categorization of the voltage transformers into groups based on their measuring signals, a status of the voltage transformers and, if applicable, of the switchgear can be estimated solely by a relative comparison of the measuring signals from spatially separated voltage transformers without knowledge of the correct voltages. Due to the occurrence of several groups, it can be established that there are inconsistencies between the measured values of the voltage transformers and an appropriate message is produced for the attention of the control system or the operating personnel. The method is independent of the configuration or the network scheme of the primary devices of the switchgear and, in particular, is independent of the complexity of their networking and can thus be implemented for arbitrary schemes without significant adaptations. The method is also independent of the instantaneous operating condition of the switchgear and, in particular, can even be carried out when the target voltage is varying.
The tracking of the topology occurs dynamically, i.e. the topology or the division into sub-areas is updated continuously or on demand. In particular, it can change owing to switching operations. Each sub-area represents a topological part of the switchgear, which has a characteristic target voltage and an actual voltage deviating therefrom in the case of system faults. It is also advantageous that use can be made of measured signals of the voltage transformers that are already available in the substation or switchgear control system. The method is also compatible with conventional local plausibility tests for voltage transformers. In contrast to these, it allows even small deviations or errors in the scaling of voltage transformers to be detected. Furthermore, more serious faults on the primary side or defects in voltage transformers can also be detected.
In a preferred exemplary embodiment, when the indication or alarm signal is present, a number of voltage transformers in each group is determined and the following criteria for judging the state of the voltage transformers are used: (i) If exactly two groups are present and if the first group contains exactly one voltage transformer and the second group contains more than one voltage transformer, then the voltage transformer in the first group is identified as being problematic and is assigned an alarm message, and/or (ii) If at least two groups of equal size or more than two groups are present, then all of the voltage transformers are identified as being problematic; and/or (iii) If a largest group with several voltage transformers and several groups each with exactly one voltage transformer are present, then all of the voltage transformers in the largest group are identified as being accurate or faultless and the other voltage transformers are identified as being problematic. These rules for judging the condition or state are very simple and nevertheless allow a reliable checking of the voltage transformers and the switchgear with little computational effort and without additional measuring effort related to the voltage transformers. In particular, it is not necessary for a correct or an absolute voltage level to be known or to be estimated.
In a further exemplary embodiment, there are further criteria for judging the state of the voltage transformers and/or of the switchgear: (i) If exactly one larger and exactly one smaller group of voltage transformers are present and if the smaller group has more than one voltage transformer, then the sub-area of the switchgear is identified as being problematic; and/or (ii) If exactly one group with several voltage transformers is present, which have a non-vanishing voltage or the averaged group voltage of which is not equal to zero, then all of the warning messages and/or alarm messages for the voltage transformers in the group or in the sub-area and, where applicable, for the sub-area itself are reset.
The exemplary embodiments as claimed in claim 4 have the advantage that the seriousness of the underlying fault in the case of problematic voltage transformers can be estimated and brought to the attention of the operating personnel.
The exemplary embodiments as claimed in claim 5 represent simple solutions for the presentation of the results of the plausibility test in an manner that is intuitively easily to understand.
The exemplary embodiment as claimed in claim 6 has the advantage that faults both in voltage transformers and in the switchgear itself can be recognized.
The exemplary embodiment as claimed in claim 7 has the advantage that it is possible to differentiate between inconsistencies between voltage measuring signals caused by switching operations and inconsistencies caused by actual faults.
The exemplary embodiment as claimed in claim 8 has the advantage that several phases can be monitored at the same time with little effort.
In further aspects, the invention relates to a computer program product and a computer program for the plausibility checking of voltage transformers, the method steps as claimed in claims 1-6 being implemented by program code and, furthermore, a device for carrying out the method and a switchgear comprising the device.